Roller Finger Followers (RFF) are widely used in overhead cam internal combustion engines for the sequential opening and closing of the cylinder intake and exhaust valves. In a typical application, the RFF and mating parts, which are part of the engine's valve train system, mechanically link the valves to the lobes of the overhead cam. Through this linkage, rotational motion of the lobes of the cam is translated to axial motion of the valves causing the valves to open and close as required to complete the combustion process.
In normal operation, the RFF must withstand the engine's environment and thus is designed to withstand the forces required to open and close the valves rapidly and repeatedly over the life of the engine. The RFF must also withstand the extreme internal temperatures of the engine, and operate with low friction to minimize wear on the moving parts and to optimize engine efficiency. The RFF is comprised of three major parts--a roller bearing, a transverse roller bearing shaft, and a finger which holds the roller bearing and shaft in fixed relation to the overhead cam lobe. The roller bearing can be of a solid, journal type, or of a needle bearing type having typically 12 to 18 needle bearings positioned between the bearing's internal diameter and the outer diameter of the shaft. Both types are known in the art. Unless otherwise indicated, the term "roller bearing" will apply to both types. The shaft, and roller bearing which is rotatably mounted to the shaft, is received in first and second coaxially aligned bores formed in the side walls of the follower finger, perpendicular to the longitudinal axis of the finger. Once the roller bearing and shaft are installed in the finger in this fashion, the ends of the shaft are mechanically deformed, such as by orbital staking or the like as known in the art, to keep the shaft and bearing in place, radially, and to prevent undesirable axial movement of the shaft relative to the finger.
The mating parts to the RFF include a hydraulic lash adjuster (HLA) member, a valve stem, and a lobe of the overhead cam. The HLA member compensates for thermal expansion and contraction in the valve train and for valve seat wear using internal hydraulic pressure from the engine's lubricating system, as known in the art. In its closed position, the sealing head of the valve is biased against the valve seat by a valve compression spring, which exerts a closing axial force on the valve stem to assure reliable sealing between the valve head and the valve seat. In order to open the valve, the movement of the RFF must overcome the spring's closing force, and it must do so repeatedly.
In operation, on the bottom side of the RFF, a first end of the finger is in contact with the axially moveable valve stem while the opposite second end of the finger is in contact with the stationary HLA member. On the top side of the RFF, with the roller bearing and shaft in position between the first and second ends of the finger, the lobe of the overhead cam is in contact with the roller bearing. By hydraulically compensating for any variation in lash, the HLA member always remains in contact with the second end of the finger, to assure a continuous contact between the cam lobe and roller bearing.
As is well known in the art, under engine operation, as the cam rotates, the lobe portion of the cam stays in rotational contact with the roller bearing to exert a rotational force on the roller bearing and a downward force on the RFF. Since the stationary HLA member in contact with the second end of the finger acts as a fulcrum, the downward force exerted on the roller bearing by the rotating cam lobe translates to a downward force exerted by the first end of the finger on the valve stem to open the valve against the closing force of the valve spring.
As the cam lobe rotates, it remains in contact with and rotates the RFF's roller bearing. The roller bearing rotates on the shaft's cylindrical surface. Since the shaft ends are mechanically staked in place in the bores of the follower finger, there is no relative rotational movement between the shaft ends and the finger bores. Thus, all relative rotational movement within the RFF is between the cylindrical surface of the shaft and the roller bearing.
To reduce wear on the shaft, and to extend the life of the RFF, it is known in the art to fabricate the shaft out of steel and to harden the cylindrical surface of the shaft on which the roller bearing rides through, for example, an induction hardening process. As mentioned above, the roller bearing and shaft are kept in place in the finger by mechanically deforming or staking the ends of the shaft after the shaft is properly positioned in the axially aligned bores. Therefore, to accommodate the staking operation, it is incumbent to localize hardening of the shaft to only that portion of the cylindrical surface of the shaft in contact with the roller bearing. That is, in order to accomplish the mechanical deformation or staking operation, the ends of the shaft must be kept relatively soft. Conversely, the hardness of the cylindrical surface of the shaft in contact with the roller bearing should be relatively hard to resist wear.
A problem with known roller shaft design is that the special handling of the shaft to localize the hardening process is expensive and difficult to maintain. To avoid hardening of the ends of the shaft, the ends must be masked off, selectively softened at the ends, or in some way isolated from the hardening process. Variations in the ability to control localization of the hardening process can lead to excessively hard shaft ends or unacceptably soft areas in the bearing contact zone. This, in turn, could cause either inadequately staked shaft ends which would permit undesirable axial movement of the shaft relative to the follower finger, or cause cracks in the shaft ends from the staking operation. These cracks could loosen the shaft and could lead to premature wear of the RFF. Conversely, variations in the ability to control localization of the hardening process can lead to an under hardened cylindrical surface of the shaft in the bearing contact zone resulting in excessive shaft wear from the rotating roller bearing. In addition, since the dimensional tolerances of the bores in the side walls of the follower finger must be closely controlled in order to assure proper fit between the shaft and the bores and proper alignment of the roller bearing with the cam lobe, careful machining of the finger in order to receive the shaft is a tedious and expensive operation. Further, the staking operation requires extreme care to avoid distorting the walls of the finger that could cause the roller bearing to bind. In addition, once the shaft staking operation is completed, the RFF cannot be readily disassembled. Therefore, should the RFF be found inoperable after the staking operation is completed, the RFF cannot be easily repaired. Finally, in the present design, the selection of a suitable shaft material must accommodate opposing needs. The shaft material has to be hard enough to resist wear from the rotating roller bearing, yet soft enough to be staked in place in the finger to prevent axial movement of the shaft. Thus, material selection for the shaft must be compromised to accommodate these opposing needs.
Also, because of the competing requirements for softness at the shaft ends but hardness in its center, a transition zone of some length is produced. The width of this zone constrains the allowable material thickness of the side walls of the finger requiring it to fully cover the transition zone.